Horner identified the former world champions as the Germanâs main rival, closely followed by his McLaren team-mate Lewis Hamilton , Ferrari’s Fernando Alonso and Kimi Raikkonen of Lotus.

But while admitting Vettel faces a fight to add a third straight F1 crown to his name, Horner warned all contenders that there is more to come from the 24-year-old.

Back-to-back champ: All eyes will be on Vettel this season (Picture: AP)

âWe are yet to see the best from him,” Horner told BBC Radio 5 live.

âHe will evolve and get stronger.â

âSeb had an unbelievable year and in the end dominated significantly.

âBut there are some big opponents out there, with six world champions on the grid.

âLewis is going to be pushing hard this year, Jenson had a great year last year, you can’t rule out Fernando because he is a world-class driver and it will be interesting to see how Kimi fares on his return.â

Looking ahead: Hamilton tipped to be a key part of the chasing pack this season (Picture: Action Images)

When asked to pick out the strongest rival to Vettel and his Red Bull team-mate Mark Webber, Horner replied: âProbably Jenson.â

Button finished second in the 2011 season, but never seriously troubled the German, who claimed the driversâ title by 122 points.